Step 1.     Loosen the strings slightly, so that you won't scratch the head while you are moving the
bridge around. Then using a yard stick or ruler, measure from the nut to the middle of the 12th
fret.  If you don't have a yardstick handy, you can use a piece of wood, and mark it at the 12th fret.
 
 

Step 2.    Now measure that same distance from the 12th fret to the banjo head,, and make a small
pencil mark on the head at that distance, and place the bridge there.  It's now in the general location.
 

Step 3.    Tune the instrument to standard tuning, using your ear, or an electronic tuner.
 
 

Step 4.    Now it's time to fine tune the placement. Play each string open, and then freted at the 12th
fret. Using your ear, determine if the fretted note is higher or lower than the open note.  If the fretted note
is higher, (sharp) the bridge is too close to the neck and needs to move toward the tailpiece.  If the fretted
note is lower, (flat) the bridge needs to be moved toword the neck.  If you measured correctly in step 2,
these movements should be very slight.  Possibly only 1/16 of an inch or so.